a) Field of the Invention
The present invention relates to a process for the continuous production of a high-strength and high-modulus polyethylene material having a modified surface. More specifically, the present invention provides a process for continuously producing a high-strength and high-modulus polyethylene material having adhesive properties by causing a specific olefin polymer to exist on or in ultra-high-molecular-weight polyethylene powder or layer upon compression molding the polyethylene powder and/or upon rolling the compression-molded layer.
b) Description of the Related Art
Polyolefins having extremely high molecular weights, namely, so-called ultra-high-molecular weight polyolefins have found wide-spread commercial utility in various fields, as unique engineering plastics having not only excellent impact resistance and abrasion resistance but also self-lubricating properties. Since these ultra-high-molecular weight polyolefins have molecular weights far higher than general-purpose polyolefins, they are expected to be able to furnish molded products having high strength and high tensile modulus provided that they can be highly oriented. High orientation of the ultra-high-molecular weight polyolefins has hence been investigated in various ways.
However, compared with general-purpose polyolefins, ultra-high-molecular weight polyolefins have higher melt viscosity and when molded or otherwise processed by conventional methods, exhibit much poorer moldability and processability. Under the circumstances, they cannot not be highly oriented by stretching.
According to a conventional process for the high orientation of an ultra-high-molecular weight polyolefin, the ultra-high-molecular weight polyolefin is formed into a dilute solution in a solvent such as xylene, decalin or kerosine. The dilution solution is cooled and isothermally crystallized, whereby a single-crystal mat is obtained. This single-crystal mat is then subjected to solid-phase extrusion, stretching, etc. Even with this process, the problem that a great deal of a solvent is required upon production of a single-crystal mat still remains unsolved.
With a view to overcoming the problem described above, the present inventors proposed a process for the production of a high-strength and high-modulus polyolefin material, in which powder of an ultra-high-molecular weight polyolefin is compression-molded at a temperature lower than the melting point of the powder without dissolution or fusion, followed by rolling and stretching (Japanese Patent Application Laid-Open Nos. 41512/1988 and 66207/1988). The present inventors then came up with some improvements to the compression molding step of the above-mentioned process, on which Japanese Patent Application No. 320401/1988 was filed.
On the other hand, materials such as fibers and sheets, which are obtained by subjecting ultra-high-molecular weight polyethylene to high orientation have high strength and tensile modulus. They are therefore expected to find a wide variety, including single use, lamination, and combination with different kinds of materials into composite materials.
Even if bonding of a high-strength and high-modulus polyethylene material with an epoxy resin or unsaturated polyester which is generally used as a base resin for composite materials is attempted with a conventional adhesive so as to provide a composite material, no sufficient bonding strength is obtained as the polyethylene material does not contain polar groups. On the other hand, fusion bonding of high-strength and high-modulus materials for their lamination must be conducted at a temperature lower than the melting point. Such a low temperature however can provide only low bonding strength, resulting in the problem that satisfactory laminate or composite material cannot be obtained.